This invention relates to a coating dispersion for the production of a support layer for catalytically active components on exhaust gas catalysts. The catalysts include an inert structure-reinforcing element with the coating dispersion coated thereon. The invention also relates to a process for the production of the dispersion and to a monolithic catalyst coated with the dispersion. The coating dispersion includes an aqueous dispersion of one or more temperature-resistant support materials as solids and, optionally, one or more other solids and/or one or more dissolved compounds as promoters and/or active components.
The pollutants in exhaust gases, particularly in the exhaust gases of internal combustion engines of motor vehicles, are a health hazard to human beings, animals and plant life. Accordingly, the pollutants have to be converted as completely as possible into harmless compounds by treatment of the exhaust gases. The pollutants are, in particular, unburnt hydrocarbons, carbon monoxide and oxides of nitrogen.
Exhaust gases have been successfully treated with multifunctional catalysts. Provided that the combustion process is suitably controlled, these multifunctional catalysts are capable of converting a high percentage of the pollutants into the harmless reaction products carbon dioxide, steam (water) and nitrogen.
The catalysts required for this purpose have to meet stringent requirements with respect to light-off performance, effectiveness, long-term activity and mechanical stability. For example, when used in motor vehicles, the catalysts must become active at low temperatures, and, in the long term, must guarantee a high percentage conversion of the pollutants to be removed in all the temperature and space velocity ranges in question.
At present, monolithic catalysts have been used as well as bead catalysts. Monolithic catalysts include either an inert metallic honeycomb or an inert, low-surface ceramic molding permeated by several parallel passages. The ceramic material may be, for example, cordierite, mullite or .alpha.-aluminum oxide. Moldings of cordierite are the preferred. This material has a favorable thermal expansion coefficient so that the support has good thermal shock properties. These properties are required to accommodate the rapid changes in temperature in catalytic converters of vehicles. A temperature-resistant layer is applied as support for the active catalyst components to the structure-reinforcing element of the monolithic catalyst. This support layer usually includes a mixture of an optionally stabilized, high-surface area aluminum oxide of the transition series, and one or more promoter oxides such as, rare earth oxides, zirconium oxide, nickel oxide, iron oxide, germanium oxide and barium oxide. A suitable stabilized aluminum oxide is described in German patent DE 38 39 580, which is entirely incorporated herein by reference.
The active catalyst components are usually metals of the platinum group, such as platinum, palladium and/or rhodium, wherein the ratio by weight of platinum and/or palladium to the rhodium optionally present is 1:1 to 30:1, according to DE-OS 38 30 318, which is entirely incorporated herein by reference.
The catalysis-promoting high-surface area support layer is applied by coating techniques known to those skilled in the art. To this end, a temperature-resistant, catalysis-promoting support material of high specific surface area (approx. 50 to 250 m.sup.2 /g) is applied by dipping the catalyst element into an aqueous dispersion of the support material (or "washcoat") or into a solution of the salt which can be thermally converted into the support material. After removal of excess dispersion or solution and subsequent drying, the coated catalyst element is calcined at temperatures of generally above 450.degree. C. This procedure may have to be repeated several times to obtain the desired layer thickness.
Basically, the same process is also used to coat flat and corrugated metal foils (cf. Finnish patent 75 744, which is entirely incorporated herein by reference) which are subsequently further processed to honeycomb-like shapes by rolling or forming stacks of foils and introducing them into tubes, or by fixing, for example by means of axial rings or metal pins (cf. Finnish patent application 89 6294, which is also entirely incorporated herein by reference). Catalyst bodies produced in this way are used for exhaust emission control in the same way as catalytically coated perforated metal foils, for example according to DE-OS 39 39 921 or DE-OS 29 42 728, each of which are entirely incorporated herein by reference.
The catalytically active noble metals can be applied to the high-surface area support layer by the following two different methods.
In the first method, the particles of the coating dispersion are completely or partly impregnated before coating the catalyst element by addition of an aqueous solution of one or more soluble compounds of the noble metals to the dispersion. Subsequent coating of the catalyst element with the dispersion thus prepared gives a support layer in which the active components are uniformly distributed.
In the second method, the catalyst element is first coated with the coating dispersion. After drying of the layer, it is impregnated, for example, by immersion of the catalyst element in an aqueous solution of the noble metal compounds. In general, the active components are not uniformly distributed in the support layer thus impregnated. The concentration is high at the surface and decreases towards the bottom of the layer. By suitably controlling the impregnation process, the degree of inhomogeneity can be controlled and, hence, optimally adapted to the catalytic process.
To activate the catalyst, the noble metal components are normally reduced in a hydrogen-containing gas stream at temperatures of 250.degree. to 650.degree. C.
Basically, any of the temperature-resistant high-surface area support materials typical of catalysts and also their "precursors" may be used. Thus, the catalyst element may be coated with an aqueous dispersion of at least one compound from the group consisting of: oxides of magnesium, calcium, strontium, barium, aluminum, scandium, yttrium, the lanthanides, the actinides, gallium, indium, silicon, titanium, zirconium, hafnium, thorium, germanium, tin, lead, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten. Furthermore, at least one member from the group consisting of: the carbides, borides, silicides and nitrides of the transition metals may be used as the support material. Hydroxides, carbonates, oxide hydrates, hydroxyl carbonates, oxalates, citrates, acetates and other readily decomposable compounds may serve as precursors of these materials.
Temperature-resistant support materials which synergistically enhance the effect of the actual catalytically active components are preferably used. Examples of such support materials are simple and composite oxides, such as active aluminum oxide, zirconium oxide, tin oxide, cerium oxide or other rare earth oxides, silicon oxide, titanium oxide, or silicates, such as aluminum silicate, or titanates, such as barium or aluminum titanate, and zeolites.
The various phases of active aluminum oxide of the transition series, which may be stabilized in accordance with DE 38 39 580 by doping with silicon oxide and lanthanum oxide and also with zirconium oxide and cerium oxide, have proven to be particularly successful temperature-resistant support materials. These support materials may be mixed or doped with promoters which, for example, increase the oxygen storage capacity of the catalyst as a whole. Suitable promoters are, in particular, the oxides of cerium, iron, nickel and/or zirconium. They have a favorable effect on the long-term activity of the catalyst and, in addition, afford advantages where the pollutants of internal combustion engines are simultaneously oxidized and reduced in a single catalyst bed.
Firm adhesion of the support layer to the catalyst element is essential to a long useful life of the catalyst. This is necessary given the rough conditions in which the catalyst is used in a motor vehicle, with its severe mechanical loads and constantly changing temperatures. In the case of a dispersion coating, the adhesion of the layer to the catalyst element is generally better if the solids of the coating dispersion are fine. Coating dispersions with particle sizes of the solids in the range from 1 to 15 .mu.m are now state of the art. In this way, firmly adhering support layers approximately 5 to 200 .mu.m thick can be applied to the catalyst bodies. A typical coating dispersion of this type is described in DE-PS 25 38 706, which is entirely incorporated by reference. The coating layer includes aluminum oxide and cerium oxide, both components having particle sizes below 3 .mu.m. Another example of conventional coating dispersions is found in EP 0 073 703, which also is entirely incorporated herein by reference. This document describes coating dispersions having a very narrow particle size distribution in the range from 1 to 15 .mu.m. To improve the adhesion of the dispersions, a binder of aluminum oxide hydroxide (for example boehmite, pseudoboehmite) or aluminum hydroxide (for example hydrargillite) is added.
The increasingly more stringent requirements of legislation, particularly the new Californian limits, necessitate further improvements in the catalysts.
In view of the test cycle (US-FTP 75) on which the new limits are based, a distinct improvement is required, particularly in light-off performance throughout the life of the catalyst. This is because, when the catalyst is warm from use, improvements are difficult to achieve on account of the high conversion rates typically reached even at the present time.
The careful handling of resources also calls for optimal utilization of the quantities of noble metals used. Accordingly, it is desirable to find coatings for catalysts which, for the same input of noble metals, show better activity than conventional catalysts.
According to EP 0 119 715 (which is entirely incorporated herein by reference), conversion rates can be increased in the case of homogeneously impregnated support layers by replacing 1 to 20% of the fine-particle solids of the coating dispersion with coarse-particle inactive material having a particle diameter of at least 44 .mu.m and a relatively high percentage of macropores. In this proposed solution, the fine-particle solids are impregnated with the catalytically active noble metals before the coating dispersion is prepared, while the coarse-particle inert material remains unimpregnated. The function of the coarse-particle inert material is merely to bring the exhaust gases to be treated into better contact with the noble metal components uniformly distributed over the depth of the support layer via the macropores.
The success of this measure in improving light-off performance is questionable because the high-surface area solids valuable to the catalytic process are partly replaced by low-surface area material of no value to the catalytic process.
The coarse-particle material does not participate directly in the emission control process. This material first has to be heated by the exothermic reactions taking place on the catalytically active solids. As a result, heating of the catalyst to its operating temperature is slowed down so that the light-off performance of the catalyst is impaired.
There is an upper limit to the margin for improving catalytic activity by the method of EP 0 119 715, namely, for the same quantity of coating, any improvement in the diffusion of exhaust gases to the bottom of the support layer with increasing percentage content of the coarse-particle inactive material is precluded by a reduction in the catalytically active, fine-particle material. Accordingly, the percentage content of coarse-particle material (by weight) in the support layer is limited to at most 20%. Although the quantity of active aluminum oxide in the catalyst could be increased again by greater layer thicknesses, this would inevitably result in an increase in the backpressure and hence to a loss of performance of the engine. In addition, on account of the greater layer thickness, noble metal would also be deposited at greater depths together with the fine-particle material. The noble metals would therefore be more inaccessible to the gaseous pollutants. This would neutralize the advantages of improved exhaust diffusion by coarse-particle inert material. According to EP 0 119 715, the coarse-particle material is produced from reject catalysts which are said to be sensibly disposed of in this way. However, this has proven to be unfavorable in practice because the highly calcined catalyst bodies of cordierite or corundum lead to the premature wear of the grinding and coating tools on account of their high abrasiveness.
In addition, highly calcined, compact materials of the type in question tend to sediment in the coating dispersions. Thus, even where there are minor differences in the treatment of the coating dispersion (i.e., uneven stirring), this may result in differences in viscosity and, hence, to uneven coating results. For these reasons, the process in question has never been successfully adopted in practice.